In general, a revolution per minute (RPM) of an air blower increases or decreases to allow a state of charge (SOC) of a battery to become closer to, or approximate, a constant target value.
That is, in a case that the SOC of the battery does not converge to the target SOC value, the RPM of the air blower increases to perform a charge operation such that the SOC of the battery reaches, or approaches, the target SOC value. However, when the SOC of the battery is rapidly increased to exceed the target SOC value, the RPM of the air blower decreases to reduce the SOC of the battery.
Under this circumstance, when the RPM of the air blower is reduced, the SOC of the battery is rapidly decreased, thereby repeatedly causing frequent charging and discharging. Referring to FIG. 1, the charge and discharge of the battery occurs frequently.
When the generation of an electric energy is not as efficient as in the initial state due to changes in a state of health of the fuel cell stack, the number of charge and discharge cycles becomes greater since a relatively small amount of electric energy is generated with the same amount of air blown. As the number of unnecessary charge and discharge cycles increases, an efficiency of an entire battery system is deteriorated. Also, different drivers have different demands for rapid charges and discharges, and slow charges and discharges. However, with the conventional technology, it is difficult to control the charges and discharges depending on a driver's propensity.